Figured out the answer. The trick was:
@topic.linkers.build.build_article
That builds the linkers, then builds the article for each linker. So, in the models:
topic.rb needs accepts_nested_attributes_for :linkers
linker.rb needs accepts_nested_attributes_for :article
Then in the form:
<%= form_for(@topic) do |topic_form| %>
...fields...
<%= topic_form.fields_for :linkers do |linker_form| %>
...linker fields...
<%= linker_form.fields_for :article do |article_form| %>
...article fields...